New Developments in Diffusion QMC for materials

We have applied the many-body ab-initio diffusion quantum monte carlo (DMC) method to calculate the band gap of ZnO and to study the oxygen vacancy in this material. DMC calculations clearly rule out the oxygen vacancy as the source of the persistent n-type conductivity in ZnO. The DMC results were compared with Hartree-Fock, the Heyd-Scuseria-Ernzefhof (HSE) hybrid functional and other approximations of density functional theory (DFT). DMC predicts the band gap at 3.43(9) eV. DMC and HSE show that the thermodynamic transition levels of the oxygen vacancy are deep, between 1.8 and 2.5 eV from the valence-band maximum. The oxygen vacancy is unlikely to be the source of the persistent n-type conductivity in ZnO, confirming previous DFT calculations. Despite this agreement between DMC and HSE, we found two major differences: (i) the oxygen vacancy formation energy is about 1 eV higher in DMC than in HSE and other DFT approximations and (ii) DMC predicts a positive U behavior for the oxygen vacancy while HSE and other DFT approximations predict the opposite. These results are discussed in conjunction with recent experiments.

Physical Review Materials

We present diffusion Monte Carlo (DMC) results for equation of state and quasiparticle gaps of manganese binary oxides MnO and MnO2 and the ternary oxide LaMnO3. Owing to the limited approximations made and the direct treatment of electronic correlations, our DMC-based study correctly describes structural properties such as the lattice constant, bulk moduli, and cohesive energies. It correctly predicts the ground-state phase of these oxides, which have different valences. Our study demonstrates the capability of DMC methods to predict the structural properties of highly correlated systems, which have been identified as a suitable candidates for many applications ranging from catalysis to electronic devices. Our study also serves as a benchmark for both the manganese pseudopotential and other methodological choices to be used in calculations of similar oxides.

The Journal of Chemical Physics, 2015

We have applied the many-body ab-initio diffusion quantum Monte Carlo (DMC) method to study Zn and ZnO crystals under pressure, and the energetics of the oxygen vacancy, zinc interstitial and hydrogen impurities in ZnO. We show that DMC is an accurate and practical method that can be used to characterize multiple properties of materials that are challenging for density functional theory approximations. DMC agrees with experimental measurements to within 0.3 eV, including the band-gap of ZnO, the ionization potential of O and Zn, and the atomization energy of O 2 , ZnO dimer, and wurtzite ZnO. DMC predicts the oxygen vacancy as a deep donor with a formation energy of 5.0(2) eV under O-rich conditions and thermodynamic transition levels located between 1.8 and 2.5 eV from the valence band maximum. Our DMC results indicate that the concentration of zinc interstitial and hydrogen impurities in ZnO should be low under n-type, and Zn-and H-rich conditions because these defects have formation energies above 1.4 eV under these conditions. Comparison of DMC and hybrid functionals shows that these DFT approximations can be parameterized to yield a general correct qualitative description of ZnO. However, the formation energy of defects in ZnO evaluated with DMC and hybrid functionals can differ by more than 0.5 eV.

Physical Review X, 2014

In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground-state properties of strongly correlated materials.

Phys. Chem. Chem. Phys., 2016

Spin density surfaces in the low temperature phase of Ti4O7: the ferromagnetic state (left panel), and the lowest-energy antiferromagnetic state (right panel). The figures were generated using self-interaction corrected density functional theory.

Journal of physics. Condensed matter : an Institute of Physics journal, 2018

QMCPACK is an open source quantum Monte Carlo package for ab initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow type trial wave functions in conjunction with a sophisticated optimizer capable of optimizing tens of thousands of parameters. The orbital space auxiliary field quantum Monte Carlo method is also implemented, enabling cross validation between different highly accurate methods. The code is specifically optimized for calculations with large numbers of electrons on the latest high performance computing architectures, including multicore central processing unit (CPU) and graphical processing unit (GPU) systems. We detail the program's capabilities, outline its structure, and give examples of its use in current research calculations. The p...

Physical Review B

We present a many-body diffusion quantum Monte Carlo (DMC) study on the ground and excited state properties of crystalline CoO polymorphs. To our knowledge, DMC is the only electronic structure method available to provide correct energetic ordering within experimental error bars between the three CoO polymorphs: Rocksalt, Wurtzite, and Zincblende. We compare these results to density functional theory (DFT) using state-of-the-art functionals such as SCAN. For the structural properties, such as the lattice parameters and bulk moduli, our results are comparable to HSE and SCAN. Using DMC, we calculated the indirect and direct optical gaps as 3.8(2) and 5.2(2) eV. Our indirect optical gap compares well with the conductivity measurements of 3.6(5) eV and GW calculations with 3.4 eV. Similarly, we obtained the DMC indirect and direct quasiparticle gaps as 3.9(2) and 5.5(2) eV. DMC direct quasiparticle gaps compare well with the direct band gap of 5.53 eV obtained from ellipsometry studies.

Physical Review B, 2010

We investigate the binding of single and quadruple hydrogen molecules on a positively charged Ca ion. By comparing with benchmark quantum Monte Carlo (QMC) calculations we demonstrate wide variability in other more approximate electronic structure methods including common density functionals. Single determinant QMC calculations find no binding at short range by approximately 0.1 eV for the quadruple hydrogen molecule case, for a fixed hydrogen bond length of 0.77 Angstrom. Density functional calculations using common functionals such a LDA and B3LYP differ substantially from the QMC binding curve. We show that use of full Hartree-Fock exchange and PBE correlation (HFX+PBEC) obtains close agreement with the QMC results, both qualitatively and quantitatively. These results both motivate the use and development of improved functionals and indicate that caution is required applying electronic structure methods to weakly bound systems such as hydrogen storage materials based on metal ion decorated nanostructures.

Physical Review Materials

NiO is a canonical Mott (or charge-transfer) insulator and as such is notoriously difficult to describe using density functional theory (DFT) based electronic structure methods. Doped Mott insulators such as NiO are of interest for various applications but rigorous theoretical descriptions are lacking. Here, we use quantum Monte Carlo methods, which very accurately include electronelectron interactions, to examine energetics, charge-and spin-structures of NiO with various point defects, such as vacancies or substitutional doping with potassium. The formation energy of a potassium dopant is significantly lower than for a Ni vacancy, making potassium an attractive monovalent dopant for NiO. We compare our results with DFT results that include an on-site Hubbard U (DFT+U) to account for correlations and find relatively large discrepancies for defect formation energies as well as for charge and spin redistributions in the presence of point defects. Beyond fitting to a single property, it is unlikely that single-parameter tuning of DFT+U will be able to obtain accurate accounts of complex properties in these materials. Responses that depend in subtle and complex ways on ground state properties, such as charge and spin densities, are likely to contain quantitative and qualitative errors.

Scientific Reports

Oxygen defects are essential building blocks for designing functional oxides with remarkable properties, ranging from electrical and ionic conductivity to magnetism and ferroelectricity. Oxygen defects, despite being spatially localized, can profoundly alter global properties such as the crystal symmetry and electronic structure, thereby enabling emergent phenomena. In this work, we achieved tunable metal–insulator transitions (MIT) in oxide heterostructures by inducing interfacial oxygen vacancy migration. We chose the non-stoichiometric VO2-δ as a model system due to its near room temperature MIT temperature. We found that depositing a TiO2 capping layer on an epitaxial VO2 thin film can effectively reduce the resistance of the insulating phase in VO2, yielding a significantly reduced ROFF/RON ratio. We systematically studied the TiO2/VO2 heterostructures by structural and transport measurements, X-ray photoelectron spectroscopy, and ab initio calculations and found that oxygen va...